Pakistani police say seven aid workers have been killed in a drive-by shooting near the north-western town of Swabi.

The workers were on their way home from a shift at a children's community centre when four men approached their van on motorbikes and sprayed it with bullets.

Among the dead were six women - five of them teachers - and a man who worked as a health technician.

The driver of the van was seriously injured.

Last month nine polio vaccination workers were shot dead in a string of attacks in Karachi and north-west Pakistan.

Those killings prompted the UN children's agency and the World Health Organisation to suspend work on polio campaigns in the country.

Staff at the community centre say they had not received any threats but that a vaccination program could be the reason its staff were targeted.

There has been no immediate claim of responsibility, but police say they are investigating possible links to Islamist militants.

Charity workers condemned the attack and called for protection.

"Schools and NGOs have been threatened in the recent past. Several government schools had been bombed in the last several months," said Rooh ul-Amin, who heads an umbrella organisation of charities in Swabi.